High-speed video observations of a +CG flash caused by a sequence of bidirectional leaders forming a ground-reaching branch of pre-existing in-cloud channel

Abstract:

High‐speed video and electric field change data were used to analyze the initiation and propagation of four predominantly vertical bidirectional leaders making connection to a predominantly horizontal bidirectional leader channel previously formed aloft. Observations were performed at the Tall‐Object Lightning Observatory in Guangzhou (TOLOG), China. The four bidirectional leaders sequentially developed along the same path and served to form a positive branch of the horizontal in-cloud channel, which became a downward positive leader producing a 135-kA positive cloud-to-ground (+CG) return stroke. The positive (lower) end of each bidirectional leader elongated abruptly at the time of connection of the negative (upper) end to the pre-existing channel aloft. Additionally, we observed twenty-six negative streamer-like filaments (resembling recently reported “needles”) extending sideways from the positively charged horizontal channel in response to the injection of negative charge associated with the +CG flash. These side (radial) branches decayed soon after extending from the horizontal channel and some of the decayed side branches were reilluminated by one or more recoil leader or streamer type events. The side branches were observed within 0.48 to 2.02 ms after the onset of +CG return stroke and extended away from the horizontal channel over approximately100 to750 m at speeds of approximately 5 to 19 ×10⁶ m/s. This study can help to improve our understanding of one of the initiation mechanisms of +CG flashes that involves downward branching of in-cloud lightning channels.